Biopsy specimens were obtained from the bronchial or the nasal mucosa of three patients with grass pollen-induced bronchial asthma or rhinitis 48 h after positive bronchial or nasal provocation test with grass pollen extract. T cell clones (TCC), derived from these and control specimens, were then assessed for their phenotype, allergen-specificity, profile of cytokine secretion and ability to provide B cell help for IgE synthesis. Out of 50 and 61 CD4+ TCC derived from the bronchial mucosa of the two patient with atopic asthma 11 (22%) and 19 (31%), respectively, showed both proliferation and cytokine production in response to grass pollen allergens under major histocompatibility complex-restricted conditions. Of these 21 (70%) exhibited a clear-cut type 2 T helper (Th2) profile and induced IgE synthesis in autologous peripheral blood B cells in the presence of grass allergens. All the other 9 grass-specific clones showed a Th0 pattern of cytokine secretion, but only 1 provided moderate help for IgE synthesis. In contrast, the majority of TCC, derived under the same experimental conditions from the bronchial mucosa of two nonatopic patients with toluene diisocyanate-induced asthma, were CD8+ and most of them produced interferon-gamma or interferon-gamma and interleukin-5, but not interleukin-4, in response to nonspecific stimulation. Of 22 CD4+ TCC3 (14%) derived from the grass-stimulated mucosa of the patient with allergic rhinitis, but none of those derived from the unstimulated nostril of the same patient, exhibited proliferation and cytokine production in response to grass allergens. All had a clear-cut Th2 profile and provided help for IgE synthesis by autologous B cells. These data indicate that inhalation of the relevant allergen results in the activation of allergen-specific Th2 lymphocytes in the airway mucosa of patients with allergic respiratory disorders. These cells may play a central role in determining the nature of the inflammatory response in the airways of atopic patients.